This invention relates to cathode ray picture tubes used in projection television systems, and is particularly concerned with means for optimizing the performance of such tubes.
Projection television systems typically include at least one cathode ray picture tube having a cathodoluminescent screen on the inside surface of the face panel. Electron-beam generating means disposed on the cathode ray tube electron-optical axis provide for forming an electron image on the cathodoluminescent screen. This electron image is converted to a visible image by the screen. Projection lens means on the projection optical axis of the tube provide for projecting the aerial image of the visible image onto a viewing screen whereon the viewer sees the television picture. The viewing screen may be of the front-projection type, or of the rear-projection type wherein the aerial image is projected onto the side of the screen opposite the viewer. To provide for compactness of the projection system, the path of the aerial image is normally "folded" by means of one or more mirrors.
Projection television systems may have a bank of red, green and blue image source means including three cathode ray picture tubes each with an associated projection lens for projecting into coincidence a red, green, and blue image to form a composite color image on the viewing screen.
A desirable--indeed necessary--feature of a projection television system is the ability to project an image of adequate brightness on the viewing screen. Brightness preferably should be equal to that of the typical shadow mask color picture tube which provides an average brightness of 80 foot-Lamberts at a beam current of 1.5 milliamperes, and with a peak brightness potential of about 320 foot-Lamberts. In view of the relatively long projection path and consequent effect of the inverse-square law, this brightness objective has proved difficult to achieve in projection television systems.
The face panel of a cathode ray picture tube used in projection television systems can be circular, with a diameter of about six inches. Alternately, the face panel can be of rectangular configuration, with dimensions of approximately 4.5 inches in height, and 5.5 inches in width, by way of example. The visible image that is electron-formed on the cathodoluminescent screen on the inner surface of the face plate is a rectangle of three to four aspect ratio. To provide a projected image of four feet in diagonal measure having a brightness of eighty foot-Lamberts, for example, the brightness of the image on the cathodoluminescent screen of the projection tube must be in the range of eight thousand to nine thousand foot-Lamberts.
An undesired byproduct of image brightness of this magnitude is the undesirably high temperatures which are developed in the tube envelope, especially in the face panel area, as a result of the electron bombardment of the face panel. For example, the operating temperature of the surface of the panel may vary between 80 degrees Centigrade and 90 degrees Centigrade with 85 degrees Centigrade being considered a practical limit at room ambient. As the cathode ray tube envelope is made of glass, the envelope is prone to thermal cracking at higher temperatures, especially in the area of the imaging screen of the face panel where the temperature differential is greatest.
One way to obviate the thermal cracking tendency is to reduce the power consumption of the cathode ray tube from an optimum fifteen watts, for example, which provides a projected picture of acceptable brightness, to about eight watts, wherein the projected picture exhibits a marginally acceptable brightness. This obviously self-defeating measure has proved necessary in some prior art projection systems to provide an acceptable operating life.
So a major factor in limiting the brightness of a projection television system is the thermally induced cracking of the face panel.
Even if the face panel does not actually crack as a result of heat, the high temperatures concentrated in the small imaging area can produce other undesired effects. For example, high face panel temperature can affect the associated projection lens, which is normally located closely adjacent to the face panel. The composition of such a lens is usually a plastic which provides the benefits of light weight and lower cost; however, the plastic is heat deformable. If the face panel reaches temperatures in the 90-100 degrees Centigrade range, the plastic may deform and destroy the focusing properties of the lens.
High face panel temperatures can produce another undesired effect known as "thermal quenching," wherein the light output of the phosphor falls off with increase of temperature beyond a certain limit. Elevated temperatures adversely affect the light output of both the green-light-emitting and the red-light-emitting phosphors, with the condition being particularly acute with respect to the green phosphor.
A coolant-sealed cathode ray tube for projection color television was disclosed by Sony Corporation at the IEEE Chicago Spring Conference on Consumer Electronics, June 4, 1981. The CRT front panel is composed of two flat glass plates, one for the face panel and one for the screen panel. A mixture of ethylene glycol and water is sealed between the two panels, making cooling contact primarily with that part of the face panel through which the image passes, and with the immediately adjacent area. The face panel and screen panel are held in relative adjacency by a cast aluminum bracket which also acts as a separator for the two panels. A temperature-vulcanized silicone is used as a sealant and for bonding the panels to the bracket. The benefit attained is said to be a lowering of face panel temperature by as much as 20 degrees Centigrade during operation.